A bearing is used to lower friction of a mechanical driving unit in various fields. For example, it is known that over one-hundred bearings are used in an automobile. Improvement in performance of the bearings directly leads to improvement in power performance of an engine and contributes to energy reduction resulting from lowering of a frictional force.
The present inventors discovered in 2003 that control of crystalline orientation of ZnO coating developed lowering of a frictional force at a nano level due to Coulomb repulsion caused by a piezoelectric effect (Non Patent Literature 1) and clarified in 2008 that the low-friction phenomenon occurred under a vacuum environment at a macro level as well (Non Patent Literature 2). The present inventors thereafter succeeded in developing this low-friction phenomenon in oil as well (Non Patent Literature 3). Since ZnO is an oxide, ZnO is excellent in chemical stability, can be used under a high-temperature and high-humidity environment, and is almost harmless to a human body. Thus, ZnO is advantageous in that an influence of scattering of abrasion powder on a living environment can be reduced.
The bearing is one of core parts used to support the shaft. Breakage of even one of the bearings in a device can, cause not only a stop of the device but also serious destruction and ignition of the device. In this manner, although the bearing is required to have high reliability, the bearing is often used for a long period under severe conditions such as high load, high-speed rotation, and high temperature. Accordingly, a method for forming firm ZnO coating highly uniformly over the entire spherical surface of the rolling element is one of core techniques essential to achievement of a low friction bearing with ZnO coating.
However, the ZnO coating disclosed in the aforementioned Non Patent Literatures is one provided on a planar surface, and the Non Patent Literature does not disclose a method for forming ZnO coating on the entire spherical surface such as a rolling element of a bearing. When coating is to be provided on surfaces of an object in all directions including the upper, lower, right, and left directions, coating the object while the object is being floated in free space is not realistic unless the cost and the processing speed can be ignored, and the object to be coated thus needs to be coated in a state of contacting another object. However, this causes a problem in which coating such a contact point and portions around the contact point is prevented from being formed. Thus, forming coating while a contact point is moved is conceivable. In this case, however, friction and collision with another object occurring on the surface of the object at the time of moving the contact point cause damage of the coating which is in the middle of formation and is not firm enough, which is an obstacle to formation of uniform and firm coating. Further, it is considerably difficult to provide a coating substance uniformly to the surfaces in all directions and keep other coating conditions for the entire surfaces evenly.
Patent Literature 1 describes a rolling element depositing method invented by the present inventors, which is not ZnO coating. In this depositing method, a holder formed in a coil spring shape or a mesh shape is prepared, and deposition is performed while the holder housing a spherical or cylindrical object is rotated. Patent Literature 1 discloses that a coil-spring-like holder is used, and that MoS2 is successfully coated on the entire surface of a cylindrical bearing rolling element by means of magnetron sputtering.
However, MoS2 is known as a substance which has long been used as a solid lubricant, which can form a coating film for friction lowering on a surface of an object extremely easily, and which can be coated so easily that adjustment of sputtering conditions can substantially be dispensed with in a case of coating by means of sputtering. As for MoS2, which can be coated under highly variable sputtering conditions, a large number of experiments described in Non Patent Literature 4 shall be referred to, for example. Hence, it is apparent to those skilled in the art that there is no guarantee of enabling a method for coating MoS2 to be applied to ZnO coating, which has more strict coating conditions than the MoS2 coating.
In addition, as is apparent from a commercially available lubricating sprayer using an oily liquid in which MoS2 particles are dispersed, attaching the MoS2 particles to the surface of an object by means of sliding is widely performed as one of methods for forming the MoS2 coating film. Thus, contact and collision of the cylindrical bearing rolling element to be coated with the coil-spring-like holder or another bearing rolling element occurring in the process of coating formation performed in an embodiment in Patent Literature 1 may be rather advantageous to MoS2 coating. However, unlike the MoS2 coating, since ZnO coating cannot be formed by means of sliding, it is natural to regard contact and collision of the surface to be coated with another object during the ZnO coating as being disadvantageous in consideration of the possibility of damage of the ZnO coating, which is still fragile in the middle of formation. Furthermore, as described in the aforementioned Non Patent Literatures, it is important to control crystalline orientation of the ZnO coating to use the ZnO coating for friction lowering. Physical disturbance in the process of coating formation such as the contact and the collision may inhibit the necessary control of the crystalline orientation and be disadvantageous to achievement of coating with uniform crystalline orientation.